1. Field of the Disclosure
The present disclosure relates to a polylactic acid-based decorative body that satisfies the performance requirements of electronic equipment cases.
2. Background Art
In recent years, from the standpoint of environmental protection, studies have been made on the reduction of the use of petroleum-derived materials and on alternative use of plant-derived materials.
Examples of petroleum-derived materials include synthetic resins such as polycarbonate resins, polycarbonate resins/acrylonitrile-butadiene-styrene copolymers (PC/ABS), and aromatic nylons. These have advantages such as lightweight, high strength, and long life and are used in various fields. However, these have the problem of high environmental loading, because when incinerated, these produce intense heat to cause damage to incinerators and release a large amount of carbon dioxide. When disposed by landfill, these cause lack of disposal site or landfill, because these have high volume occupancy and are not degradable in the soil. Also when dissipated into the natural world, these have an adverse effect on wild animals and so on to cause the problem of environmental destruction. The raw material petroleum is a resource whose reserves are limited, and therefore, plant-derived plastic materials have been studied as alternative materials.
Among plant-derived plastic materials, particularly, polylactic acid resin, which is made from plants such as corn and sweet potato, undergoes hydrolysis in the soil to be reduced in molecular weight and is finally decomposed into carbon dioxide and water by microorganisms. Polylactic acid resin is also characterized in that when it is incinerated, the amount of heat generation is relatively small, and the amount of carbon dioxide release is also relatively small. In addition, polylactic acid resin has been attracting attention as a low-environmental-loading material, because the raw material plants absorb carbon dioxide in the course of their growth.
Polylactic acid resin also has characteristics such as high rigidity, relatively high tensile strength, and high transparency, and molded products thereof have begun to be applied in various fields such as food containers, horticultural sheets, electronic equipment cases, and automobile parts (see for example Patent Document 1). There are many examples in which synthetic resin coating compositions containing synthetic resin such as acrylic resin or urethane resin are applied to the surface of molded synthetic resin products so that surface conditions such as esthetic appearance and protection against scratches are improved. Also for molded products of polylactic acid resin, coating compositions for adding some functions have been aggressively developed, and there have been proposed a pressure-sensitive adhesive (see for example Patent Document 2) and a coating agent (see for example Patent Document 3) each having good adhesion to a polylactic acid resin substrate.
3. Discussion of the Background Art    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2008-150560, paragraph 0029    Patent Document 2: JP-A No. 2004-231797    Patent Document 3: JP-A No. 2006-291000
However, polylactic acid resin tends to be inferior in heat resistance, impact resistance, chemical resistance, and weather resistance to synthetic resin particularly for use in durable consumer goods. Under wet and hot conditions, polylactic acid resin is also reduced in molecular weight by hydrolysis, so that its strength is reduced, and therefore, the development of the application of polylactic acid resin alone is restricted. For example, when it is used for electronic equipment cases such as cases of personal computers and cellular phones, not only it should withstand long-term use, but also it should satisfy heat resistance, impact resistance, water resistance, chemical resistance, and surface performance requirements. In addition, molded products of polylactic acid resin may be deformed by heat generated from electronic equipments, or their surface may be affected and deteriorated by sweat, hand cream, or the like on the hand of the user, and therefore, solutions to these problems are necessary.
Patent Document 1 proposes a polylactic acid-based resin composition that is produced by adding a flame retardant to polylactic acid resin and can be used as an industrial material, but it does not solve the problem of high hydrolyzability which is a disadvantage of polylactic acid resin. Moreover, the inventors have tried to improve hydrolysis resistance by forming a coating film on the surface of a polylactic acid resin-containing substrate. The results have shown that conventional synthetic resin coating compositions comprising petroleum-derived materials have insufficient adhesion to substrates and that polylactic acid resin coating agents such as those disclosed in Patent Document 3 cannot solve the problem of high hydrolyzability although they have good adhesion to substrates, so that the performance requirements of electronic equipment cases cannot be satisfied using the existing coating compositions.